IV PREFACE. 



ciples on which the action of the instrument depends 

 (which will assist the reader to understand the opera- 

 tion of its constituent parts), including a sketch of 

 the subject of polarized light. The order in which 

 the subjects are treated is scientific, and particular 

 directions have been given for the examination of 

 the objects. 



The small size of the work has necessitated the 

 exclusion of figurative descriptions, so that it is 

 adapted rather for a worker than a reader ; at the same 

 time, the matter forms a course, and must be taken 

 as a whole for the proper comprehension of the sub- 

 jects. The technical terms used are referred to in 

 the Index, so as to furnish to some extent a glossary 

 of terms; and their derivation is given, to facilitate 

 their recollection. The figures, with very few excep- 

 tions, are drawn from nature, and are coloured that 

 the objects may be more easily recognized. The 

 magnifying powers under which they have been drawn 

 are denoted by a small number placed beneath each 

 figure : and the particular attention of the reader is 

 requested to this point ; otherwise the whole subject 

 will be utterly confused ; so much does the appearance 

 of objects vary under different powers. 



Directions are given for preparing and mounting 



